camera: add snapshot support for libcamerasrc and qtiqmmfsrc

[nitinn22]

This PR enhances the camera validation suite with snapshot pipeline coverage.
Changes are split into two logical commits:

Camera tests: snapshot support for both libcamerasrc and qtiqmmfsrc
GStreamer helper: supporting snapshot pipeline construction for qtiqmmfsrc

[smuppand]

Enabling the snapshot by default in the existing Camera_Tests flow. This changes the behavior of the current LAVA test from fakesink/preview/encode to fakesink/preview/encode/snapshot, adds jpegenc/multifilesink dependency coverage, and introduces a different still-capture validation path into the existing pre-merge test.

Suggested approach:

• keep run.sh/lib_gstreamer.sh support for snapshot
• keep Camera_Tests default as fakesink,preview,encode
• add a separate Camera_Snapshot_Tests.yaml that runs the same run.sh with CAMERA_TEST_MODES=snapshot

This gives us snapshot coverage without disturbing the existing camera test flow.

Also please clean up the help text so the default test count remains 10/7, with snapshot adding 2 optional tests, and remove/log the currently unused gstRc variable in the snapshot functions.

[nitinn22]

Hi @smuppand , snapshot is more of default test mode in camera which i added and it does not have to be separate optional entity, i added few functions to handle them which does not disturb existing implementation ideally, let me know if it does so. Its preferred to run all the 4 modes as mandatory and none are optional for camera imo, earlier we didn't have L0 for still snapshot only mode which we have now.

[nitinn22]

Addressed comments, dead code removed, size constant added and removed redundancy, error handling fixes from skip to fail for snapshot validations, preflight check added for snapshot test mode.

[smuppand]

LGTM